The present invention generally relates to a heat roller fixing arrangement for use in an electrophotographic copying apparatus, and more particularly, to a heat roller fixing device which includes a heating roller formed by laminating an offset prevention layer on an electrically conductive core member, and a pressure roller constituted by forming an electrically insulative layer on another electrically conductive core member for passing between said heating roller and pressure roller a copy paper sheet which carries a toner image formed thereon by the known electrophotographic method so as to fix the toner image onto the copy paper sheet, and especially, to a heat roller fixing device of a type having no offset prevention agent on the surface of the heating roller.
The heat roller fixing device of the above described type has such advantages that copy material, for example in the form of a copy paper sheet is provided with a good feel to the hand of the user after the fixing and which does not soil the hand of the user with an offset prevent agent, and that, since no means is required for applying the offset prevention agent, the fixing device may be simplified in its construction, with a consequent reduction in cost, etc., as compared with heat roller fixing devices which employ the offset prevention agent. On the contrary, however, the heat roller fixing device of the above described type which does not employing the offset prevention agent has a drawback in that the undesirable offset phenomenon tends to take place very easily. Conventionally, various means have been proposed for preventing the offset phenomenon, for example, by employing a heating roller coated with a fluorine resin layer having superior release properties and heat resistance, as an offset prevention layer for the improvement of releasing between the heating roller and toner, or by proper selection of binder resins for the toner or addition of release agents such as low-molecular weight polypropylene and the like for the improvement of release properties of the toner itself. However, in the known fixing devices of the above described type, it has still been difficult to completely prevent the offset phenomenon unless the offset prevention agent is employed.
Moreover, in the heating roller laminated with the fluorine resin as an offset prevention layer, there is a problem that the surface thereof is abraded or damaged by the contact with copy paper sheets, a copy paper separating claw, a temperature control element or the like. Particularly, in the case where the offset prevention layer is made thin for better heat conduction, the core member or core metal of the heating roller may undesirably be exposed, with an extreme reduction in the release properties, thus giving rise to the offset phenomenon. Accordingly, the offset prevention layer is required to have a thickness in the range of 30 to 40.mu., but in this case, heat conduction is reduced, with a consequent reduction in stability of temperature on the heat roller surface.
Although there have conventionally been proposed some heating rollers in which pigments are mixed in the fluorine resin, or fluorine resin powder is blended into a binder having favorable abrasion resistance, these known heat rollers still have drawbacks in that the release properties thereof as fixing rollers are inferior, with a tendency to produce the undesirable offset phenomenon.